In many modern image display systems, image frames of an analog image signal, such as analog video, analog computer graphics, analog DVDs, analog game console output, and so forth, may be digitized prior to being displayed. Digitizing the image frames of the analog image signal may enable processing of the image frames by image processing hardware in the image display system. The processing performed by the image processing hardware may improve image quality, reduce image noise, enhance desired properties of the image, deemphasize undesired properties of the image, and so on, for example.
The digitizing may include sampling the analog image signal without prior knowledge of a sampling frequency or sampling phase using an analog to digital converter (ADC). In order to properly digitize the images in the analog image signal, a sampling frequency generally must be detected. If the sampling frequency is incorrectly detected, then when the digitized images are displayed, the resulting images may be distorted at best or completely incomprehensible at worse.
In order to correctly detect the sampling frequency, a sampling frequency offset may need to be detected. The sampling frequency offset generally is an offset between an actual sampling frequency of the analog image signal and a sampling frequency of a sampling clock used to time the digitizing of the analog image signal. The presence of a sampling frequency offset may result in the appearance of a vertical interference pattern on images displayed by the image display system. FIG. 1a illustrates an image frame 100 from an analog image signal. The image frame 100 includes a light grey background. FIG. 1b illustrates a displayed image frame 150. The image frame 150 may be representative of the image frame 100 sampled with an incorrect sampling frequency (with a non-zero sampling frequency offset). The non-zero sampling frequency offset may create an aliasing effect that is visible on the displayed image frame 150 as a vertical interference pattern. The vertical interference pattern may include vertical bars, such as vertical bars 155 and 156.
A period of the vertical bars, such as a period between the vertical bars 155 and 156 shown as interval 160, in the vertical interference pattern may be inversely proportional to the non-zero sampling frequency offset. It may be possible to determine the non-zero sampling frequency offset by measuring the period of the vertical bars. Please refer to a co-assigned U.S. patent application entitled “Method and Apparatus for Analog Graphics Sample Clock Frequency Offset Detection and Verification,” U.S. Patent Publication Number 2006/0274194 A1, filed Feb. 16, 2006, which patent application is hereby incorporated herein by reference, for a detailed discussion of the determination of the sampling frequency offset from the period of the vertical interference pattern.